Telephone installations where a plurality of telephone lines, Private Branch Exchange (PBX) extensions or a combination of telephone lines and PBX extensions that are connected directly to a single telephone set are referred to as key telephone systems, key systems or keyphone systems. Such systems are intended for use primarily in small companies, large residences, and in departments of larger organizations where a PBX is used. A keyphone system offers greater convenience, flexibility and control than a single PBX extension, and on premises where no PBX is provided, a key system obviates the need to have two or more telephone sets on each desk where a plurality of lines are used.
Keyphone systems are generally more expensive than an equivalent number of single-line telephone sets, especially in installations that require from two to five telephone lines and/or PBX extensions. Thus, the cost to the subscriber becomes an important factor in determining whether to install such a system.
One reason for the high cost is that the system includes a central control unit that must be set-up and installed at a location remote from the telephone sets. The control unit involves a substantial cost, both for the control unit itself and for the auxiliary wiring that must be attached between the control unit and each of the telephone sets. The telephone sets themselves are also expensive since they are especially made to accommodate the maximum number of key functions for which the system has been designed.
Conventional keyphone systems additionally involve a much larger maintenance allocation for total life cycle costs than do single line telephone sets. With the latter, or with residential sets, public attitudes and policies now accept over-the-counter delivery of customer equipment, installation by the user via modular jacks, and trouble isolation by the user that obviates expensive service calls where user may carry-in his set to a repair facility that supports detachable sets.
The complexity of conventional keyphone systems has discouraged extension of this economy to such keyphones.
Although there are other telephone systems available that provide a form of two-line service at a relatively low cost, none of these existing two-line systems has the capability or offer the full convenience and flexibility of keyphone systems.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention. However, the following U.S. patents are considered related and indicative of the state-of-the art:
______________________________________ U.S. Pat. No. INVENTOR ISSUED ______________________________________ 4,169,217 Szanto, A. 25 September 1979 4,132,860 Rasmussen, H. 2 January 1979 4,011,413 Phillips, R. 8 March 1977 ______________________________________
The Szanto patent discloses a line status apparatus that is used with telepone sets not having built-in telephone line status indicators. The apparatus comprises a line sensing circuit including a transducer that is applied across the Tip and Ring leads of a telephone line. The circuit/transducer generates an output while any terminal attached to the line is off-hook. The output, in turn, is used to activate a visual indicator that indicates the off-hook condition.
The Rasmussen patent discloses a hold control that allows a two-line, multiple telephone set installation to be converted into a keyphone system. The hold control unit is connected in series between the telephone central office lines and the telephone set. The telephone lines are connected to separate inputs provided at each control unit at each station so that telephone calls can be received or initiated over either of the lines using the telephone set at that station.
Each control unit includes a hold circuit which is responsive to a hold switch, and that provides for establishing a hold condition on one of the lines over which a telephonic connection has been made through the telephone central office. The hold circuit connects a line terminating impedance across the line that is to be held and concurrently applys a hold condition indicating signal onto the line by periodically varying the line terminating impedance. The impedance establishes a condition on the line that simulates and is interpreted by the central office as an off-hook condition which causes the central office to maintain the connection with the remote party. The hold condition is automatically terminated when the held line assumes an actual off-hook condition.
The Phillips patent discloses an electronic hold circuit for a single line telephone. The circuit provides a single electronics sensing and latching circuit connected across the telephone lines. The circuit responds to the voltage across the lines, and operates in conjunction with a plurality of hold actuating circuits that apply a secondary, not normally encountered, voltage on the lines where such voltage causes the latching circuit to switch the line to a hold condition. The latching circuit remains quiescent in response to the relatively high voltage on the phone lines in the on-hook condition and the relatively low voltage in the off-hook condition, and responds to an intermediate hold signaling voltage for latching the line in a hold condition.